The invention relates generally to an apparatus and method for determining an angular position of a rotating component. More particularly, the invention relates to an apparatus and method for high resolution angular position sensing of permanent magnet rotors.
Control methods for conventional permanent magnet AC motors use the angular position of the rotor to control the electrical phase and frequency of the stator excitation currents. Failure to maintain the proper electrical phase relationship results in loss of torque production and reduced efficiency.
Various methods are used to determine the angular position of the rotor. For example, the electromotive force (EMF) of the machine is measured by sensing voltage at normal operating speeds. This method, however, is not effective at lower speeds or at stall because the EMF is small or zero. Incremental encoders are commonly used with induction machines but are not as useful with permanent magnet machines because the phase position of the magnets is not known at startup. Brushless DC motors and drives often use a set of three Hall-effect sensors to determine the angular positions of the permanent magnets. However, the angular resolution provided by the sensors is low, i.e., the position signal provides only six output states per magnet pole pair.
External sensor methods utilize components such as resolvers, optical encoders, and notched target wheels. These techniques are generally used to achieve high resolution for accurate control of position or low speed torque. External approaches are often costly, bulky and fragile. In addition, external sensors can require a large number of noise-sensitive signal wires and complex signal decoding techniques.
Accordingly, there exists a need for a simple, robust apparatus and method that determine the angular position of rotor magnets with high resolution for a range of operating speeds from stall to the maximum motor speed. The apparatus should provide a simple electrical interface for integration with other electrical components and systems. The present invention satisfies these needs and provides additional advantages.
In one aspect, the invention features an apparatus for determining an angular position of a rotating component having an axis of rotation and having a plurality of angularly spaced magnetic elements. The apparatus includes a sensor board and a processor circuit. The sensor board is disposed adjacent to the rotating component and includes a plurality of sensor groups. Each of the sensor groups includes a plurality of magnetic sensors positioned to sense a magnetic field of one of the angularly spaced magnetic elements. Each of the sensor groups generates a respective multi-state group signal in response to the passage of one of the angularly spaced magnetic elements by the respective sensor group. The processor circuit is in communication with each of the sensor groups and generates at least one position signal in response to the multi-state group signals. In one embodiment, the magnetic sensors of each sensor group are configured in parallel electrical communication.
In another aspect, the invention features a method of determining the angular position of a rotating component having a plurality of angularly spaced magnetic elements. The method includes generating, for each of a plurality of magnetic sensors in each of a plurality of sensor groups, a two-state signal responsive to the position of at least one of the angularly spaced magnetic elements of the rotating component relative to the magnetic sensor. The method also includes combining, for each of the sensor groups, the two-state signals generated by the respective magnetic sensors to generate a respective multi-state group signal and comparing the multi-state group signals to determine the angular position of the rotating component.
In another aspect, the invention features an apparatus for determining an angular position of a rotating component having a plurality of angularly spaced magnetic elements. The apparatus includes a sensor board having a plurality of magnetic sensors and a plurality of resistive elements. The magnetic sensors are configured in parallel electrical communication. Each magnetic sensor has a first terminal adapted to receive a first reference voltage and a second terminal. Each magnetic sensor electrically communicates the first terminal with the second terminal in the presence of a local magnetic field of a predetermined polarity. Each resistive element has a first terminal in electrical communication with the second terminal of a respective magnetic sensor and a second terminal adapted to receive a second reference voltage. In a further embodiment, the apparatus includes a channel circuit. The channel circuit has a first terminal in electrical communication with the second terminals of the resistive elements. The first terminal of the channel circuit is also in electrical communication with the first terminal of a sense resistor. The channel circuit has a second terminal in electrical communication with a second terminal of the sense resistor and a third terminal adapted to provide a voltage signal that indicates the number of magnetic sensors in the presence of the local magnetic field of the predetermined polarity. The voltage signal is responsive to a voltage difference between the first and second terminals of the channel circuit.